1. Field of the Invention
The present invention relates to a method of estimation of traffic information and a device of estimation of traffic information for estimating traffic information of roads from which no traffic information is being acquired from traffic information of roads from which the traffic information has been acquired and to a car navigation device for calculating a route by using the traffic information estimated by the method of estimation of traffic information or the device of estimation of traffic information.
2. Description of Related Art
In recent years, it has become possible for a car navigation device to guide a route corresponding to a traffic status at a given time or to a pattern of changes of a traffic amount of a day by using real-time traffic information or statistical traffic information acquired by statistically processing the real-time traffic information provided from traffic information providers.
However, the traffic information provided from the traffic information provider is normally that of expressways and trunk roads and many times, no traffic information of arterial roads is given. As a consequence, the traffic information of such arterial roads, e.g., a link travel time, is handled as what does not change throughout a whole year or whole day. In such a case, the car navigation device is unable to calculate a route to be guided such as a route of shortest time accurately corresponding to a traffic status during commute time for example.
It is noted that a road network is supposed to be composed of nodes and links, wherein the node corresponds to a crossroad such as an intersection and the link corresponds to a road connecting two crossroads. In case of an expressway, its entrances, exits and interchanges correspond to the nodes.
The traffic information of such road network includes the link travel time described above, link speed and others for example. The link travel time is a time required for a vehicle to travel a certain link and the link speed is a value acquired by dividing a length of the link (distance) by the link travel time. Because the traffic information is often found by correlating with links in general, it is called specifically as link traffic information in such a case.
JPH10-283591A discloses an exemplary traffic information estimating method for estimating traffic information of a link having no traffic information by taking a weighted mean of traffic information of a link having the traffic information. The traffic information estimating method assumes such that the larger a distance between links and a difference of directions of the links, the smaller the weight is when the weighted mean is taken. That is, traffic information of a link having no traffic information is calculated by relying on a neighboring link closest to own link and a link oriented in the same direction as much as possible among the links having traffic information.
There has been also a technology of specifying and guiding a recommended route based on traffic information by a navigation device that guides the route by calculating routes from a present location to a destination. However, the traffic information is not provided for all roads and is limited only to main roads. Therefore, there has been proposed a technology of a navigation device as described in JP2005-122461A as a technology for complementing also traffic information of roads for which no traffic information nor statistical information is provided based on traffic information and statistic information of neighboring roads.
JP2005-122461A describes the navigation device that complements traffic information of a road for which no traffic information is provided based on traffic information of closely located roads among roads for which the traffic information is provided or on traffic information of roads within a predetermined range.
However, the traffic information estimating method disclosed in JP. H10-283591A does not consider types of roads. Therefore, in case when an expressway is mixed in a road network and when the expressway has traffic information and arterial roads around the expressway have no traffic information, inadequate information will be calculated as traffic information of the arterial roads if the traffic information of the arterial road is estimated from the traffic information of the expressway. It is unable to estimate link speed of arterial roads whose speed limit is 40 km/hr. from link speed of an expressway whose speed limit is 80 km/hr. by the weighted mean described in JP. H10-283591A. Even if it is possible to estimate the link speed, the calculated speed is not accurate. Accordingly, the traffic information estimating method disclosed in JP. H10-283591A cannot be applied to a road network mixed with an expressway.
Moreover, the navigation device as disclosed in JP. 2005-122461A complements the traffic information by averaging the traffic information of the roads closely located in the same route or the roads within the predetermined range, so that it hardly reflects traffic information of a congestion characteristic to a specific intersection where the congestion is presumed.
In view of the problems of the prior art technologies described above, there have been needs for providing a method of estimation of traffic information and a device of estimation of traffic information (referred to also as a “traffic information estimating method” and a “traffic information estimating device”, respectively, hereinafter) that allow traffic information of a link having no traffic information to be accurately estimated based on traffic information of a link having the traffic information even for a road network in which an expressway and arterial roads are mixed and for providing a car navigation device that calculates a route by the traffic information estimated by using the traffic information estimating method or the traffic information estimating device.
There has been also a need for providing a technology of a car navigation device for accurately complementing traffic information for roads for which no traffic information is provided.